To explore the effect of anisotropy and KOH corrosion process on the mechanical properties of monocrystalline silicon flexible rib structure, the morphological changes of indentation-induced cracks with respect to the crystal orientation on the three primary crystal planes were observed by nano-indentation experiments combined with atomic force microscope (AFM). The change rule of elastic modulus, hardness, fracture toughness of monocrystalline silicon on the three primary crystal planes (001), (110) and (111) with respect to crystal orientation was analyzed. Meanwhile, the influence of KOH corrosion process on the surface mechanical properties of (001) crystal plane was investigated as well. The result shows that with the variation of crystal orientation, the elastic modulus on the (001) crystal plane appears the greatest change, while the hardness and fracture toughness have no significant changes. On the (110) crystal plane, both the elastic modulus and fracture toughness show obvious changes, but the hardness possesses no significant change. However, on the (111) crystal plane, the mechanical parameters (including elastic modulus, fracture toughness and hardness) of flexible rib monocrystalline silicon have no obvious anisotropic behaviors. The directions which are sensitive to the crack propagation are determined on the three primary crystal planes of flexible rib monocrystalline silicon. The surface quality of silicon processed by KOH erosion would deteriorate and the exposed surface micro-cracks and defects would result in the decrease of hardness and fracture toughness, which should greatly weaken the actual fracture strength of flexible rib structure.